Hi Roly,
Sounds like the perfect quiver of boards.
You do need a larger sail (8.5 m2+ would do) for the Carve 145 to get the most quality TOW.
Yes, sail the smallest board and sail that keeps you planing all the time.
Don't "automatically" pick any combo.
You may be able to sail the 7.8 on all 3 boards.
I used to sail a 7.8 m2 Sailworks Retro on my Carve 99, but I weighed 150 lbs. back in those days.
6.5m2 should be sweet on the Carve 99.
What issues did you have with the 52 cm fin on the Carve 145. That fin should have been about right for your 7.8 m2.
Double straps, all the way back and all the way outboard will be the best on all 3 boards for recreational slalom (fast) sailing.
If you decide to jump the Carve 99, then a single back strap is probably better as you will land more stable with your back foot over the centerline.
Hope this helps,